A tin or tin alloy plating process for surface finish or solder applications that does not contain lead is desired by the electronics industry due to environmental considerations. However, lead-free tin or tin alloy solders deposited using electrochemical techniques have shown a tendency to grow whiskers. These whiskers can cause short-circuiting, resulting in poor reliability and failure of the electronics component. A number of theories have been postulated as to the cause of the whisker growth and detrimental solder performance. The three main causes proposed are internal stresses, grain size, and organic additives.
Recent studies have also suggested that the occurrence of whiskers in pure tin deposits can be reduced by the deposition of matte finish tin, rather than bright tin. Consequently, it has been suggested that plating bath additives that make the plating deposit bright and shiny have a tendency to add internal stress and promote whisker growth. Deposition of matte tin can be achieved using plating baths with low additive concentrations, however, to achieve the required current distribution, and therefore metal distribution, as well as desired grain size, organic additives must be added to the plating bath. These additives can break down, become incorporated into the deposit, promote whisker growth, and degrade the long-term performance of the solder.
The use of pulse plating as compared to direct current plating, for control of mechanical properties of plated deposits has been previously discussed by Knodler, who measured the mechanical properties of pulsed and direct current plated gold alloy deposits, and stated that “direct current deposits are known to be highly stressed”. Knolder showed that through the use of pulse plating, the stresses in a gold alloy deposit could be reduced, as compared to deposits from direct current plating. In addition, the decrease in stress was accompanied by a loss of brightness. Huang et. al demonstrated that internal tensile stresses in 5 micrometer-thick copper foil, electrodeposited from a copper-sulfuric acid bath containing gelatin, could be reduced by the use of forward pulse currents.
Organic additives for tin and tin alloy plating baths have been previously described and are generally incorporated in plating baths to control the grain size in the desired 1 to 8 micrometer range. These additives, while maintaining the required grain size, can increase the chance of whisker growth. Furthermore, the inclusion of organic additives in the bath requires the use of complex and often expensive equipment to control the additive concentration, and to minimize the breakdown of the additives and their incorporation into the deposit. Zhang has demonstrated that in the presence of a grain refiner and a wetting agent, a grain size of 2 to 8 micrometers in a tin deposit can be obtained using a pulse current waveform. However, the grain refiner breaks down over time and must be replenished on a regular basis. Furthermore, the effect of this plating process on the internal stresses in the deposit is unknown.
Internal stresses are present in pure tin deposits after plating as described in the prior art. As the tin deposit ages, the stresses relax and single tin crystals, or whiskers, nucleate out from the surface. These whiskers can grow to lengths of greater than 100 micrometers. To release the stresses without whisker growth, manufacturers remelt the tin deposits. However, this process causes problems with corrosion or poor solderability.
To minimize whisker growth in the solder, lead-free tin alloys such as tin-copper, tin-bismuth, and tin-silver have been proposed. While these alloys may reduce the occurrence of whiskers, there are problems associated with bath stability and bath control, which prohibit these processes from being used commercially. The deposition potentials of these three alloying elements are more noble than that of tin. This causes problems with preferential deposition of the alloying elements, variations in alloy composition, immersion plating of the alloy on the tin anodes and redox reactions between the tin and the alloying elements. To overcome these problems, complexing agents and stabilizers must be added to the bath, or the concentration of the alloying elements must be maintained at very low levels, making control difficult and expensive.